All else being equal, ectotherms should maintain body temperatures (Tbs) favourable for temperature-sensitive biological functions, such as digestion and locomotion. Physical environments in the temperate zone are often thermally variable, however, thus making it difficult to maintain optimum Tbs. Radiotelemetry and a semi-natural enclosure were used to monitor Tbs of grass snakes Natrix natrix at Canterbury, Kent, U.K. Operative temperatures (Tes) were measured using snake models (copper-pipe models) placed in a variety of microhabitats to determine availability of thermoregulatory opportunities. A modification of Huey & Slatkin's (1976) regression model, piecewise regression, was used to evaluate thermoregulatory behaviour. Grass snakes could achieve high mean Tbs (>30°C) most often during midday (08:00–20:00), but only for 55–61% of the study period. Overall, non-gravid snakes maintained higher and less variable mean daytime Tbs than gravid snakes. From piecewise regression, it was determined that grass snakes initiated thermoregulation at a Te of 38.44°C, corresponding to a Tb of 27.7°C. Three main conclusions can be drawn from this study: (1) thermoregulatory opportunities for grass snakes were limited; (2) nonetheless, when conditions were sufficiently warm, there was clear evidence that grass snakes could thermoregulate; (3) contrary to expectation, gravid females actually maintained lower and more variable Tbs than non-gravid females. The consequences of these thermoregulatory patterns for the fitness of snakes have yet to be determined.